Controlling the spectral output of a diode laser through the use of an external component is well known in the art. Typically, a wavelength (or frequency) selective element is positioned in optical communication with the optically active (gain) region of the laser so as to select a specific wavelength from the output spectrum of the laser. Once an appropriate wavelength is selected, light of this wavelength is redirected back to the active (gain) region of the laser so as to provide the positive feedback required for laser action. Wavelength (or frequency) selective elements of this type often function by exhibiting very low losses at a selected emission wavelength and exhibiting very high losses at all other wavelengths.
It will be appreciated that, for most semiconductor lasers, the output spectrum does not resemble a spontaneous emission lineshape, but rather consists of a plurality of regularly spaced, narrower lines corresponding to the various resonant (high Q) frequencies and spatial modes of the laser cavity. Thus it will be understood that various prior art external cavity devices have been developed to select a single frequency and fundamental mode from among the plurality of frequencies and modes generated by a typical diode laser.
For example, in one prior art device, the light from a diode laser is directed toward an external cavity comprising a diffraction grating. In this type of device, the various wavelengths emitted by the laser are first dispersed by the grating, and then light of a selected frequency is retro-reflected back into the laser. Thus, by varying the characteristics of the diffraction grating, a preferred laser oscillation wavelength, corresponding to perfect retro-reflection, may be selected and redirected back into the diode laser for further stimulation at that wavelength. Unfortunately, while such prior art external cavity diffraction gratings allow for relatively wide wavelength tuning, e.g., 50 nm (nanometers) at 1550 nm, and relatively narrow linewidth, e.g., less than 1 Mhz (megahertz), they are typically not well suited to miniaturization or to integration into commercially available semiconductor lasers. Such prior art devices also tend to suffer from very slow response times, e.g., approximately 1 ms (millisecond).
Electro-optical filtering devices (including LiNbO.sub.3 waveguides, semiconductor band filling devices, and the like) have also been employed for the selection of the laser oscillation wavelength. However, these devices generally possess insufficient non-linearity for wide wavelength tuning, often having tuning ranges of only about 5 nm or so.
Another well known device used to select the laser oscillation frequency is a three-section tunable distributed Bragg reflector (DBR) in an external cavity. A DBR is somewhat similar to a diffraction grating, but it does not include the dispersive elements found in conventional diffraction gratings. As with other wavelength selection devices, DBR's are often designed to reflect a specific laser oscillation wavelength with maximum efficiency. The DBR, however, acts as a band-pass mirror with extremely sharp resonance. More particularly, when such a reflector is used with a typical semiconductor laser, one of the laser's mirrors is replaced by a corrugated waveguide, i.e., the DBR grating. The periodicity and material of the corrugations are selected so as to provide a sinusoidally-varying effective index of refraction along the direction of propagation of the guided light wave. In this way, when the wavevector of the guided light wave is an integer multiple of the grating wavevector, the guided wave of light is strongly reflected. On the other hand, when the wavevector of the guided light wave is a non-integer multiple of the grating wavevector, the guided wave of light will propagate freely through the device and not be redirected back to the optically active (gain) region of the laser. Of course, it will be appreciated that the spatial modes of the laser are also a factor in DBR-controlled wavelength selection.
Unfortunately, while prior art DBR's of the type disclosed above are generally susceptible to miniaturization, and while they frequently exhibit relatively fast response times, e.g., approximately 0.5 nm/ns (nanometers/nanosecond), they are also generally not widely tunable. Prior art DBR's also often exhibit a tuning range on the order of only about 4 nm or so. This narrow tuning range is generally due to the reliance on injection-induced index changes, i.e., band filling. Also, prior art DBR's often cause sudden discrete shifting (or "hopping") between the spatial modes of the laser.
Another prior art wavelength selection device comprises a grating-assisted vertical cavity coupling. This device provides for a wide tuning range by employing small optical non-linearities in its structure and by employing a long grating period. The grating-assisted vertical cavity switches the cavity mode between two intercavity waveguides that are coupled by a carrier-injected grating so as to allow relatively broad tuning, e.g., approximately 70 nm at 1.5 microns. However, this relatively wide tuning range is achieved at the cost of poor emission linewidth, or poor modal stability, or both. In particular, because of the long grating period employed by the device, at a 50 nm tuning range, linewidths of approximately 20 Angstroms or 340 GHz (gigahertz) are often produced. It will be appreciated that, in general, all such "carrier-injected" techniques tend to suffer from linewidth broadening during tuning because of shot noise. As a result, most prior art external cavity tuning devices which provide wide tuning ranges do so at the expense of broader linewidths.
As a consequence, there is a need for a miniature external cavity tuning device which operates at high speed, is dynamically tunable over a broad range of wavelengths, and does not introduce appreciable linewidth broadening or mode hopping.